Nordin, Linda

Abstract [en]

This Master thesis work has been carried out during between June and November 2006 at the Swedish Defence Research Agency (FOI) at the Department of Laser System, in Linköping. Conventional airborne laser scanner systems extract only the first and last echo from the return laser pulse. With improved processor capacities and the ability to save more data it is now possible to save and analyse the entire waveform, in post-processing, in order to extract more characteristics of the reflecting surfaces, e.g., finding more echoes. This helps create a better understanding of the shape and characteristics of objects. In vegetation more than one echo usually occurs, since the laser pulse can for example hit the canopy, some branches and leaves and finally the ground.But some of those echoes are so small that it can be difficult to distinguish them from the noise in the signal. By using deconvolution, small, yet obvious, echoes can be amplified and more information can thus be obtained about what has reflected the laser light. Deconvolution can also break down pulses from adjacent surface that can appear as one larger pulse. In this thesis the Richardson-Lucy deconvolution algorithm is used. The EM-algorithm is used to fit the waveforms to a series of Gaussians functions. After the deconvolution of the laser return from areas with vegetations more than 75% more echoes could be extracted compared to the echoes found with the system, while for areas with buildings and roads, the deconvolution did not increase the extracted echoes significantly. More echoes was found on the ground, below the trees, which enable a more accurate model of the ground to be acquired.